This invention relates to a solar cell which converts solar energy into electrical energy. More particularly, the invention relates to a solar cell structure of the hetero-face type, and a method of manufacturing the same.
FIG. 1A is a schematic section showing a conventional AlGaAs/GaAs solar cell of the hetero-face structure, which is known as a highly efficient solar cell. In FIG. 1A, reference numeral 1 designates an n-type GaAs substrate; 2, a p-type GaAs layer formed by the diffusion of a p-type impurity, ion injection or an epitaxial growth technique; and 3, a p-type AlGaAs layer formed on the layer 2 by an epitaxial growth technique. It is well known that the solar cell thus constructed has a band-diagram shown in FIG. 1B. Further, there exists an energy difference .DELTA.E between the p-type GaAs layer 2 and the p-type AlGaAs layer 3, which is equivalent to the bandgap E.sub.g1 -E.sub.g2. The existence of this energy difference serves an important role in this solar cell.
The solar cell having the hetero-face structure of FIG. 1A is highly effective in reducing the recombination of carriers at the surface of the p-type GaAs layer 2, so that a highly efficient solar cell is accomplished. However, in this solar cell, a p-type GaAs layer 2 having a high carrier mobility is employed as a main active region. The n-type GaAs substrate 1 is of a low mobility and the amount of incident light arriving at the substrate 1 is small so that the substrate 1 is not efficiently used. In particular, the depletion layer 12 existing around the pn-junction interface scarcely contributes to the generation of photo-current. Electrons generated in response to solar light incidence mainly reside in the p-type GaAs layer 2. The electrons reach the junction by diffusion and enter the n-type GaAs substrate 1 after penetrating the depletion layer region 12 due to a high voltage gradient applied to the pn-junction. Also, holes generated in the n-type substrate 1 enter the p-type GaAs layer 2 by tracing a journey opposite to that of the electrons. In this manner, a photoexcitation current flows by the diffusion of the electrons in the p-type GaAs layer 2 and the holes in the n-type GaAs substrate 1 into the respective opposite conductivity type regions. Accordingly, the device characteristics strongly depend on the diffusion lengths of the minority carriers in the respective regions.
Further, in circumstances tending to reduce the diffusion lengths of the minority carriers, for instance, in space, where radiations such as electrons, protons and .gamma.-rays are applied to the solar cell whereby the diffusion lengths of the carriers are lowered, the efficiency may be abruptly lowered. Furthermore, as the thickness of the p-type GaAs layer 2 increases, the minority carriers generated in the vicinity of the surface of the element, if the diffusion length thereof is short, vanish before they arrive at the pn-junction so that the efficiency may be remarkably lowered.